1. Field of the Invention
The present invention relates to a signal generating device for generating a signal and, more particularly, to a signal generating device for generating a carrier signal for use in down-converting a chrominance signal.
2. Description of the Related Art
A carrier signal generating device is known as one example of a signal generating device for generating a signal. The carrier signal generating device is arranged to generate a carrier signal used in the down-conversion of a chrominance signal which is performed for recording a standard color television signal on a video tape in a video tape recorder.
FIG. 1 is a block diagram schematically showing the arrangement of a chrominance signal down-converting circuit provided with a conventional carrier signal generating device.
As shown in FIG. 1, a chrominance signal (f1=3.58 MHz) inputted through an input terminal 14 is frequency-converted in a frequency converter 15 by using a carrier signal (f2=4.32 MHz) generated from the carrier signal generating device 11.
The frequency converter 15 outputs the chrominance signal whose frequency has been converted into a frequency (f2.+-.f1) to a band-pass filter (BPF) 17 over a signal line 16. The BPF 17 extracts a signal of frequency f2-f1=743 KHz from the input chrominance signal and outputs the extracted signal through an output terminal 18.
The carrier signal (f2=4.32 MHz) is required to be a signal of single frequency free from high-frequency components of second order or higher. Accordingly, as shown in FIG. 1, a trap circuit 13 made up of a ceramics filter or the like is used to eliminate such high-frequency components from the carrier signal generated from the carrier signal generating device 11 provided with a crystal oscillator. After the high-frequency components have been eliminated from the carrier signal by the trap circuit 13, the carrier signal is supplied to the frequency converter 15.
As described above, in the conventional circuit, the carrier signal generating device 11 provided with the crystal oscillator is employed to generate the carrier signal having a stable single frequency. However, since the carrier signal outputted from the carrier signal generating device 11 contains a distortion due to the characteristics of the carrier signal generating device 11 itself, the trap circuit 13 needs to be added for eliminating such a distortion.
However, since the trap circuit 13 is made up of, for example, a ceramics filter, it is difficult to form the trap circuit 13 as a small-size element. This disadvantage hinders implementation of high-density mounting and cost reduction. In addition, if the carrier signal generating circuit 11 and the frequency converter 15 are to be formed as an integrated circuit, it is necessary to provide a terminal 12 for connection with the trap circuit 13 and the total number of terminals must be increased.